According to the terminology of rolling bearings in conformity with JIS 0104-1991, a crown-shaped retainer is defined as a retainer having prongs shaped so that rolling elements and the retainer maybe assembled together through elastic deformation. The name is derived from the shape of its outer appearance in which a plurality of prongs each protruding in an axial direction are arranged on an annular base. Further, the crown-shaped retainer belongs to a retainer formed by injection molding, die casting, sintering, or the like, that is, a molded retainer.
A ball bearing is a rolling bearing using balls as the rolling elements. As illustrated in FIG. 2, the ball bearing includes, as main components thereof, an inner race 10 having a raceway 12 formed on an outer circumference thereof, an outer race 14 having a raceway 16 formed on an inner circumference thereof, a plurality of balls 18 interposed between the raceway 12 of the inner race 10 and the raceway 16 of the outer race 14, and a retainer 20 for retaining the balls 18 at predetermined intervals in a circumferential direction. Note that, illustrations of a lubricant, a seal, and other auxiliary components are omitted herein.
The inner race 10 and the outer race 14 each have a ring shape, and the balls 18 interposed between the inner race 10 and the outer race 14 concentric with each other may roll along the raceway 12 of the inner race 10 and the raceway 16 of the outer race 14. Thus, the inner race 10 and the outer race 14 are freely rollable relative to each other. One of the inner race 10 and the outer race 14 is mounted on a stationary side, whereas the other of the inner race 10 and the outer race 14 is mounted on a rotary side. In general, the inner race 10 is mounted on a rotational shaft, whereas the outer race 14 is mounted on a housing.
Various types of ball bearing such as a deep groove ball bearing and an angular contact ball bearing are used for an automotive transmission. In electric vehicles and hybrid vehicles, high-speed motor rotation is input so that the rotary part such as the rotational shaft tends to be rotated at high speed (see paragraph 0006 of Patent Literature 1). As a result, polyamide (PA) such as polyamide 46 (PA 46) or polyamide 66 (PA 66), which is a material of the related-art retainer, cannot withstand deformation caused by a centrifugal force along with the high-speed rotation, and hence polyether ether ketone (PEEK) having high mechanical strength is used to address this issue in actuality (see paragraph 0032 of Patent Literature 1). For example, when the bearing is used for such high-speed rotation that a dmn value {product of dm (mean diameter of ball row (mm) ) and n (rotational speed of ball bearing (min−1) )} is more than 600,000, the use of a highly functional resin having high mechanical strength, such as PEEK, for the production of the retainer material may suppress the retainer deformation caused by the centrifugal force.